A work exchanger is a device that obtains energy from one stream of fluid and transfers that energy to another stream. It can be also described as a pump driven by fluid flow, most often of opposed piston/diaphragm type. Work exchangers are vital for energy recovery in reverse osmosis processes such as desalination, since by itself the RO separation is power-consuming process which becomes economically feasible if only a substantial part of the energy resident in the reject or/and permeate streams is returned back into the process.
A work exchanger system typically comprises two (or more) pressure cylinder vessels with a brine port at one end, a feed water port at the other end, and a plunger freely sliding between the ports. A system of valves connects and disconnects these ports to high-pressure brine line coming from RO modules, brine discharge line, low-pressure feed water line, and high-pressure feed water line going to the RO modules. Each pressure cylinder performs a two-stroke cycle whereby the energy of the high-pressure brine is transferred to the stream of feed water. The resulting low-pressure brine is discharged.
At the first stroke, the brine port is connected to the brine discharge line while the feed water port is connected to the low-pressure feed water line. The vessel is filled with low-pressure feed water which displaces the plunger towards the brine port and brine is discharged through the non-pressurized discharge line.
At the second stroke, the brine port is connected to the high-pressure brine line, while the feed port is connected to the high-pressure feed water line. The vessel is filled with high-pressure brine which displaces the plunger back towards the feed port so as to squeeze feed water into the high-pressure feed water line.
The operation of the work exchanger requires special timing, reliable synchronization and sealing of the valves in order to perform efficiently the above two-stroke cycle.
A report on experimental work “A Flow Work Exchanger for Desalination Processes”, Kansas state Univ., Manhattan, August 1968, discloses usage of pilot-operated Hunt double plunger hydraulic valve with a work exchanger. This valve has a housing with two parallel cylinder bores and four lateral ports opening into the bores. Two rigidly connected parallel plungers are movable in the bores, providing communication between the ports through specially formed channels and cavities in plunger bodies. The plungers are always in hydrostatic balance.
U.S. Pat. No. 5,306,428 to Tonner discloses a rotary valve used to direct brine to/from different work exchanger ports. The feed water stream is regulated by two check valves at each feed water port. The rotary valve of Tonner is not hydraulically balanced, which causes excessive wear on the sealing surfaces due to side loads exerted on the central rotating assembly. There are also internal and external leakage problems between the high pressure inlet and outlet ports and the low pressure drain ports. This, in turn, reduces the efficiency of the Tonner valve and imposes size limits on any such device that can be manufactured in practice.
U.S. Pat. No. 5,797,429 to Shumway suggests the usage of a five-way or four-way linear spool valve in a work exchanger system. The Shumway valve comprises two pistons connected by a rod (spool) located inside a cylinder. The cylinder has five ports: a high pressure brine inlet, a first work exchanger vessel port, a second work exchanger vessel port, and two low-pressure brine discharge outlets which may be connected. By moving the spool back and forth within the cylinder, the work exchanger ports are alternately opened and closed, and this directs flow in the proper sequence to the proper port. The feed water stream in the Shumway work-exchanger system is regulated by two check valves at the feed water port of each exchanger vessel.
The linear spool valve of Shumway is hydraulically balanced axially. As a result, the force required to move the linear spool is only that force needed to overcome the friction of the sealing surfaces associated with the pistons, which permits the driving device of the valve to be of low power. However, the Shumway valve has also leakage problems. The attempts to reduce leakage by tighter fitting of the pistons to the cylinder lead to excessive wear which seems to be an inherent problem in every spool valve device because the sealing in spool valves is not provided by positive displacement. This problem is even more aggravated in work exchangers of large capacity and power that are employed in modern desalination plants using RO technology.
Poppet valves have relatively simple design and provide very reliable sealing achieved by positive displacement. A typical three-way poppet valve comprises a valve chamber with a central port and two coaxial valve seats leading to two end ports, and a poppet body disposed in the valve chamber. The poppet body is adapted for reciprocation between two positions so that in a first position it seals the first valve seat and fluid communication is provided between one end port and the central port, and in a second position the poppet body seals the second valve seat and fluid communication is provided between the other end port and the central port. However, during the travel between the valve seats, the poppet body allows fluid communication between all three ports. The poppet valve also shuts-off and opens abruptly which may cause water hammer, and is not hydraulically balanced.